The MIL-STD-1553B data bus is a standard protocol for controlling communication between components on board a vehicle such as an aircraft or a space vehicle. In particular, MIL-STD-1553B is a protocol for a digital, command/response, time-division multiplexing data bus 100, as shown in FIG. 1. The components include a processing element called a bus controller 105 that sends commands to subsystems 115, 120 on board a vehicle. Subsystems 115, 120 include sensors, actuators, etc. Subsystems 115, 120 interface electronically with the data bus 100 via a remote terminal, where the remote terminal can be a separate line replaceable unit such as remote terminal 110 or can be part of a subsystem with an embedded remote terminal 120. The standard defines the physical medium (a shielded, twisted wire pair cable 125 in a bus topology with all components electronically interfaced to cable 125; redundant cables 130 may be used for fault-tolerance); its transmission bit rate (1 megabit/second); the types of allowed words (command words, status words, and data words); the format for words (16 bits per word, where the position of each bit conveys meaning, plus a 3 microsecond sync (i.e., 3 bits worth of time) and a parity bit for a total of 20 bit-times per word); and the defined sequences for communication. See, for example, FIGS. 2 and 3 for various types of transfer sequences and mode command sequences. Under MIL-STD-1553B, bus controller 105 always initiates commands, and the other components (e.g., remote terminal 110) respond to the commands. For example, if data is to be transmitted to remote terminal 110, bus controller 105 transmits a command (e.g., a ‘receive’ command) followed by data words to remote terminal 110. See, for example, FIGS. 1 and 2.
Because communication is relayed between bus controller 105 and subsystems 115, 120 using cable 125, bus controller 105 has to split communication between each subsystem 115, 120 according to time. For example, for the first few hundred microseconds, bus controller 105 may communicate with subsystem 115 and, for the next few hundred microseconds, bus controller 105 may communicate with subsystem 120. Under MIL-STD-1553B, after bus controller 105 sends a command to, for example, remote terminal 110, bus controller 105 waits for a response from remote terminal 110 before sending another command to, for example, subsystem 120.
However, a problem associated with MIL-STD-1553B protocol is that MIL-STD-1553B is a protocol of trust. That is, each component and terminal is expected to follow the protocol, but there is nothing about the layout (topology) of bus cable 125, 130 to prevent a component from doing something not allowed by the protocol, such as sending a message out of turn or sending an incorrectly formatted message.
Thus, an improved way to regulate protocol components may be beneficial.